The present invention relates generally to the field of frequency selective limiter circuits and signal-to-noise enhancers.
Frequency selective limiters (FSLs) are two-port devices that can selectively attenuate undesired interference, thus improving the reception of desired signals. For example, see [A. J. Giarola, Proc. IEEE, vol. 67, no. 10, October 1979 “A review of the theory, characteristics, and operation of frequency selective limiters”].
FIGS. 13A-13D illustrate the operation of a typical FSL (see [J. D. Adam, IEEE Microwave Mag., September/October 2014, p. 45, “Mitigate the interference”]). FIG. 13A illustrates the transfer function G0 of an FSL at low powers where the input power of various frequency components of a signal is below a threshold value for each frequency component. In this case the FSL acts as a band pass filter passing the frequency components. FIG. 13B illustrates the input power of various frequency components where two of the components at frequencies ω2 and ω3 have a power above the threshold value. FIG. 13C illustrates the transfer function G0 of an FSL for the input powers of FIG. 13B, where the FSL limits the output powers of the FSL for the components at frequencies ω2 and ω3. FIG. 13D illustrates the output power of the FSL for the input powers of FIG. 13B, where the FSL limits the output powers of the FSL for the components at frequencies ω2 and ω3 to be at most a threshold value. An ideal FSL should be capable of limiting the power of individual frequency components which are above a certain threshold level, without mutual interference or without affecting any other frequency components.
Known FSLs are based on nonlinear “large signal” properties of ferrites or similar magnetic materials (see, for example, [J. D. Adam, IEEE Microwave Mag., September/October 2014, p. 45, “Mitigate the interference”]). The physical mechanism that is responsible for frequency selective power limiting in magnetic materials is based on parametric coupling between the uniform magnetic spin precession (magnetostatic wave) at a signal frequency ω with spin waves at one half of the signal frequency, i.e., at ω/2.
Magnetic material components, however, suffer from major drawbacks inherent to magnetic materials in general. Such magnetic materials are bulky, heavy, incompatible with integrated circuit technologies, sensitive to temperature detuning, require permanent magnetic bias field, and are good only in a certain frequency range (due to fixed frequency of gyromagnetic precession). Nonlinear magnetic materials are also extremely expensive since the quality requirements are higher than in the case of linear materials.